docs: align docs with unified vector model and current API

[matt-edmondson]

Summary

First pass at making the documentation match the code on this branch so we can
reference it as the source of truth while working through the open issues
(#48–#61). This PR is docs-only — no source changes.

The branch had drifted significantly from the docs:

• The physics system is now metadata-driven (dimensions.json + Roslyn generator)
  but the docs still described BootstrapUnits, hand-written PhysicalDimensions.cs,
  and an "8 domains, 80+ quantities" structure that doesn't exist anymore.
• FluentValidation has been removed but fluent-validation-integration.md still
  promised an integration.
• Validation references mentioned attributes that no longer exist (IsEmail,
  IsNotEmpty, HasLength, IsUrl, IsPositive, IsInRange).
• The strategy doc still listed the four foundational design decisions as open
  questions, which meant nothing in the docs could be authoritative.

What changed

Decisions resolved (all incorporated into the docs and tracked-against in
the open issues):

1. V0 - V0 returns the same V0 of T.Abs(a - b). (Resolve open design decision for Vector0 subtraction #52)
2. Dimensionless and angular quantities have both V0 (Ratio) and V1
   (SignedRatio) bases.
3. Semantic overloads widen implicitly to base, narrow explicitly. (No tests for semantic overload conversions (Weight, Drag, Distance, Diameter, …) #55)
4. Per-dimension physicalConstraints metadata + ArgumentException-throwing
   factory guards enforce floors like absolute zero. (Vector0 quantities do not enforce the non-negativity invariant #50, Domain-specific physical constraints (absolute zero, non-negative frequency, etc.) not enforced #51)

Rewritten

• CLAUDE.md — replaced bootstrap/units narrative with the unified vector model,
  resolved decisions, and pointers to the new docs.
• README.md — corrected counts (62 dimensions / ~195 types, not "8 domains /
  80+ quantities"), removed FluentValidation links, replaced stale physics
  examples and stale attribute names.
• docs/complete-library-guide.md — single canonical user guide; merged in the
  content of library-overview.md.
• docs/physics-domains-guide.md — rewritten against the actual dimensions.json
  contents (dimension-by-dimension with vector forms and overloads).
• docs/validation-reference.md — full attribute catalogue rebuilt from the
  actual files in Semantics.Strings/Validation/Attributes/ and
  Semantics.Paths/Validation/Attributes/.

Trimmed

• docs/strategy-unified-vector-quantities.md — converted the "Open Questions"
  sections into "Resolved Decisions". Now safe to cite as the architecture spec.
• docs/advanced-usage.md — fixed stale attribute names (IsEmail→IsEmailAddress,
  etc.), FromString<T>(...) → Create(...), FormatException → ArgumentException,
  removed [ValidateWith(typeof(...))] since the attribute doesn't exist.

Added

• docs/physics-generator.md — schema for dimensions.json plus end-to-end
  walk-throughs for adding a dimension, a semantic overload, or a relationship.

Deleted

• docs/fluent-validation-integration.md (FluentValidation removed).
• docs/design-semantic-quantities.md (superseded by the strategy doc).
• docs/library-overview.md (merged into complete-library-guide.md).
• docs/examples/comprehensive-physics-examples.md (empty duplicate of the
  PascalCase file).

Net diff: +857 / −3329 across 12 files.

docs/examples/ was deferred — those will get a follow-up pass once the open
generator/runtime issues land.

Test plan

• Skim docs/strategy-unified-vector-quantities.md Resolved Decisions
  sections to confirm they're the rules you want baked in.
• Confirm docs/physics-generator.md is the right level of detail to
  reference when working through QuantitiesGenerator emits a factory for only the first availableUnit #48 (factory generation), Vector0 quantities do not enforce the non-negativity invariant #50 (V0
  non-negativity), Domain-specific physical constraints (absolute zero, non-negative frequency, etc.) not enforced #51 (physical constraints).
• Spot-check README.md / complete-library-guide.md examples against the
  generated types in Semantics.Quantities/Generated/.

🤖 Generated docs alignment to support the work queue in #48–#61.

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Generated by Claude Code